1. Field of the Invention
The present invention relates to a light measuring device employing a focus detecting device and, more particularly, to a light measuring device which is capable of measuring the brightness of an object in spots.
2. Description of the Prior Art
It is know in the prior art, such as disclosed in Japanese Patent Laid-open Publication No. 60-129732, to carry out a spot measuring using a light receiver employed in the distance measuring device. According to this prior art reference, the signal produced from the light receiver is used, alternately in a time sharing manner, for obtaining information for distance measuring and for obtaining information for light measuring. Therefore, it takes time to complete light measuring, as well as distance measuring. This will result in such a disadvantage that there may be a difference between the spot from which the distance information is obtained and the spot from which the brightness information is obtained.
To avoid this disadvantage, it is possible to provide one light receiver for obtaining the distance information and another light receiver for obtaining the brightness information. However, this results in a high manufacturing cost.
It is also known in the prior art, such as disclosed in U.S. Pat. No. 4,387,975, to use output signals from a photoelectric element array such as CCD (charge coupled device) for obtaining the brightness of the object. However, since the obtained signals are the voltage signals representing the amount of accumulated charges in the CCD cells, it is very difficult to precisely detect the brightness level. For example if the brightness signal Bv is required to be varied between 0 and 13, it is necessary to detect the voltage signal in 2.sup.13 (=8192) different levels. To meet this requirement, it is necessary to provide a photoelectric element array which can produce an output in a wide range, or to provide a voltage detecting circuit having a high resolving power. For example, in the former situation, if the voltage signal shows 10 millivolts for Bv=0, the same photoelectric element array must produce a voltage signal of 80 volts for Bv=13. Also, in the later situation, if the voltage signal shows 6 volts for Bv=13, the same photoelectric element array will produce a voltage signal of 750 microvolts for BV=13, resulting in the necessity of a voltage detector with a high resolving power. Also, due to the dark current produced from the CCD, S/N ratio will become poor particularly for detecting low voltages.
Also in the prior art, such as disclosed in Japanese Patent Laid-open Publication No. 60-202414, a focus detection device is disclosed, which includes a light receiver composed of CCD for measuring distance and a light receiver for monitoring the amount of light received by the distance measuring light receiver for controlling the integrating time of CCD. Since the monitor light receiver detects the brightness of the object with a narrow viewing angle, the monitor light receiver is used also for the spot light measuring. However, this arrangement has disadvantages that the proper light measuring can not be done if the monitor light receiver is aiming at a point outside the object to be focused, and that since the viewing angle of the monitor light receiver is very narrow, the narrow spot aimed by the monitor light receiver may incidentally coincide with a spot on the object from which a strong light is produced. Another problem is that, when the light measuring device of the above described type is employed in a camera, there may be a case when the spot measuring can not be carried out because of the special type of the objective lens mounted on the camera body.
Also, in the above-mentioned reference, Japanese Patent Laid-open Publication No. 60-129732, a camera is disclosed which includes a first light receiver for effecting the spot light measuring, a second light receiver for effecting the average light measuring, and a comparator for comparing the signals obtained from the spot light measuring and the average light measuring for detecting a back lighting condition. According to this prior art, since the light receiver provided for use in the active type focus detecting device is used as the first light receiver, there will be no two different cases, i.e., a case when the spot measuring is possible, and a case when the spot measuring is not possible. Thus, there is no need of automatic switching between the spot measuring and the average measuring.
Also, in the above-mentioned reference, U.S. Pat. No. 4,387,975, it is disclosed that the distance calculation and exposure data calculation are carried out in a time sharing manner using the data obtained from the light receiver in the focusing detecting device. It is also disclosed that the light receiver can be provided separately from the focus detecting device. However, if it is not possible to carry out the light measuring using the light receiver in the focus detecting device, only a warning operation is performed in this reference.
It is further known in the prior art, such as disclosed in U.S. Pat. No. 4,047,187, to use output signals from a photoelectric element array of a CCD type or a MOS type for obtaining, alternately in a time sharing manner, distance information and brightness information. According to this prior art reference, the signals produced from the light receiver are sequentially logarithmically compressed and converted to a digital form and are used for the focus detection calculation. After this calculation, the signals produced from the light receiver are again sequentially logarithmically compressed and converted to a digital form and are added and divided by the number of signals added to obtain an average brightness information. In this manner, the process for obtaining the distance information and the process for obtaining the brightness information are effected at different times, resulting in a long data preparation time.
It is also known in the prior art, such as disclosed in U.S. Pat. No. 4,547,676, to stop the integration in the light receiver when the output voltage obtained from the light receiver has reached a predetermined reference voltage. This is carried out by the use of a comparator.
Also, in some prior art light measuring device, a filter circuit such as a CR integration circuit is employed to reduce the ripple signals caused by the fluorescent lamp light. However, no light measuring device of charge accumulation type has been proposed which can provide a correct light amount signal even under the fluorescent lamp light.